Modern industry standards are placing increasing demands on the low temperature performance of engine oils. The low temperature performance of formulated engine oils can be improved by improving the base oil, by improving the additives used in formulating the oil or both. The low temperature properties of base oils may also be improved by using a synthetic base oil such as a poly-alpha olefin (PAO).
The low temperature properties of any oil are influenced by the presence of waxes such as long chain paraffins. These materials are thought to form wax crystals at low temperatures. These wax materials in turn adversely affect the fluidity of the oil thus causing a deterioration of low temperature properties. It is common practice to at least partially remove waxy materials from basestocks by dewaxing. Dewaxing can be accomplished by either solvent or catalytic means. Solvent dewaxing is a physical method in which waxy molecules are separated based on their solubility properties in select solvents. Catalytic dewaxing chemically converts the waxy molecules to other molecules that have better low temperature properties. Catalytic dewaxing may occur by cracking waxy molecules or by isomerizing waxy molecules.
Another approach typically used in conjunction with dewaxing is the addition of additives such as pour point depressants as part of an additive package added to the lubricating oil basestock to form a formulated oil. Pour point depressants are generally polymeric materials that improve the fluidity of an oil, i.e., they reduce the pour point. However, any given pour point depressant will have a different influence on the pour point depending on the nature of the oil in question. While a given pour point depressant may be effective in one oil, it may be ineffective in another. Thus, it is necessary to test the low temperature properties of an oil to know the influence of any given additive package containing a pour point depressant.
One method for determining low temperature pumpability of an engine oil is based on the Mini Rotary Viscometer (MRV). Other means of measuring the low temperature properties of a formulated oil include Brookfield Viscosity, Scanning Brookfield Viscosity, Cold Cranking Simulator test (CCS) and Pour Point. While these test methods may yield information about the low temperature properties of any give oil, they do not necessarily provide information as to the compositional features of that oil.
Various physical techniques have been developed to investigate the composition of crude oils and fractions thereof, including Fourier Transform infrared spectroscopy (FTIR), liquid chromatography, gas chromatography (GC), nuclear magnetic resonance (NMR), and mass spectrometry (MS). Due to the complexity of petroleum mixtures such as crudes, no technique is capable of providing precise compositional details of all the individual molecules making up the petroleum mixture.
GC/MS methods use GC to at least partially separate a mixture into components thereof and MS is then used to identify the components. Petroleum mixtures are very difficult to resolve into individual components due to the complexity of the mixtures and the similar retention times of many individual molecules under given GC conditions.
Two-dimensional gas chromatography (2D GC) is a recent technique that has been developed as a high resolution alternative to conventional GC/MS techniques. In 2D GC, a sample is subjected to two sequential chromatographic separations. The first separation is a partial separation by a first or primary separation column. The partially separated components are then injected into a second or secondary column where they undergo further separation. The two columns usually have different selectivities to achieve the desired degree of separation. An example of 2D GC may be found in U.S. Pat. No. 5,169,039.
It would be desirable if the chromatographic separation information on molecular composition available from 2D GC could be correlated with low temperature viscometric properties of formulated oils.